The present invention relates to pressure pulse generators such as the "mud siren" type used in oil industry measurement while drilling (MWD) operations. More particularly, the present invention relates to a modulator design for a MWD tool wherein sinusoidal pressure pulses are generated for transmission to the borehole surface by way of a mud column located in a drill string.
Many systems are known for transmitting data representative of one or more measured downhole conditions to a borehole surface during the drilling of the borehole. Typically, the systems employ a downhole pressure pulse generator or modulator which transmits modulated signals carrying encoded data at acoustic frequencies via the mud column in the drill string. Indeed, it is known to use coherent differential phase shift keyed modulation to encode the data, such that if a binary "one" is to be transmitted, the signal at the end of the sampling period is arranged to be one hundred and eighty degrees out of phase with the signal at the beginning of the period. If a binary zero is to be transmitted, the signal at the end of the period is arranged to be in phase with the signal at the beginning of the period.
In some of the known MWD tools of the art, the downhole electrical components are powered by a self-contained mud-driven turbine generator unit positioned downstream of the modulator. Thus, modulators of the mud siren type generally take the form of signal generating valves positioned in the drill string near the drill bit such that they are exposed to the circulating mud path. A typical modulator is comprised of a fixed stator and a motor-driven rotatable rotor positioned coaxially of each other. As seen in FIGS. 1a-1c and 2a-2c, the stator and rotor of the art are each formed with a plurality of block-like radial extensions or lobes spaced circumferentially about a central hub so that the gaps between adjacent lobes present a plurality of openings or ports which accommodate the oncoming flow stream of mud. As seen in FIGS. 1 and 2a, when the respective lobes and ports of the stator and rotor are in direct alignment (open position), they provide the greatest passageway for the flow of the mud through the modulator and hence the pressure drop across the modulator is small. When the rotor rotates relative to the stator as seen in FIG. 2a, alignment between the respective lobes and ports is shifted, thereby interrupting the flow of mud by causing it to divide as seen in FIG. 2b. Such an interruption causes the pressure drop across the modulator to rise. At a certain point, as seen in FIG. 1c, the lobes and ports of the stator and rotor take opposite positions (closed position) such that the flow of all the mud must follow a path through the modulator gap (as seen in FIG. 2c). Such an arrangement causes the pressure across the modulator to be a maximum. Thus, rotation of the rotor relative to the stator in the circulating mud flow produces a cyclic acoustic signal which travels up the mud column in the drill string and which may be detected at the drillsite surface. By selectively varying the rotation of the rotor to produce changes in the signal, a coherent differential phase shift keyed modulated pressure pulse may be achieved.
While pressure pulse generators employing rotors and stators provide MWD tools with a means for transmitting data, it has often been difficult to detect signals due to the weakness of the signals generated. The signal generated by the modulator is known to attenuate as the depth of the tool increases, and as the viscosity of the mud increases. Moreover, the only known manners of increasing signal strength are by increasing mud flow through the modulator, decreasing the flow area through the modulator, or by increasing mud density. Thus, it will be appreciated that the only known manner of increasing signal strength which may be affected by modulator flow design is to decrease the flow area of the modulator by reducing the modulator gap. However, reducing the modulator gap makes the modulator susceptible to jamming as circulation materials can become jammed between the rotor and stator. Jamming is costly as it typically stops the modulator rotation in the full closed position, thereby preventing circulation through the MWD tool and necessitating the removal of the tool from the borehole.